For the birdshot you will be fine. Personally in my 21 inch barrel model 12 20 gauge, I use 7 and a half shot but six will work. Just more pellets of sufficient size using the 7 and a half. My shortened model 12 has no choke and it is the gun I choose when my dogs need to see a bird drop. As far as the slug goes, I think cylinder will work fine. You're main issue may be point of impact or inherent accuracy of your particular shotgun. If it won't group one brand, try the rest. One may group real well, while other suck. Hope this helps.